The invention concerns an optical sensor and an optical process for the characterization of a chemical and/or bio-chemical substance.
Waveguide grating structures with and without a chemo-sensitive layer are described in the literature (refer to, e.g., EP 0 226 604 B1, EP 0 482 377 A2, PCT WO 95/03538, SPIE Vol. 1141, 192-200, PCT WO 97/09594, Advances in Biosensors 2 (1992), 261-289, U.S. Pat. No. 5,479,260, SPIE Vol. 2836, 221-234).
In EP 0 226 604 B1 and EP 0 482 377 A2, it is demonstrated, how the effective refractive index (resp., the coupling angle) of a chemo-sensitive grating coupler can be measured as a sensor signal. The sensor signal “effective refractive index” or “coupling angle” is a value which manifests a strong dependence on temperature.
Front-side in-coupling of light into a waveguide (refer to SPIE Vol. 1141, 192-200) is not practical, because a high positioning accuracy is required. In addition, the front side of the wave guide has to be of good optical quality. In PCT WO 95/03538 it is demonstrated how the absolute out-coupling angle of a mode is measured. This value, however, without referencing manifests a high dependence on temperature. In PCT WO 97/09594, chirped waveguide gratings are presented, which, however, also manifest a dependence on temperature.
In Advances in Biosensors 2 (1992), 261-289 it is shown how the disturbing “pore diffusion” can be referenced away with the three-layer waveguide model. The refractive index of the waveguiding film manifests drift, while the layer thickness of the waveguiding film (=sensor signal) remains stable. The layout is designed with movable mechanics, which does not permit rapid measurements. In addition, the sensor signal or the light emerging from the waveguide grating structure is recorded from the front side. Front side detection is unsuitable for a two-dimensional layout of waveguide grating structure units. Furthermore, the effective refractive indexes N(TE) and N(TM) for the two polarizations TE and TM are not simultaneously recorded, because for the recording of resonance coupling curves separated by angle a mechanical angular scan is carried out.
In U.S. Pat. No. 5,479,260, a bi-diffractive or multi-diffractive grating coupler is described, whereby the sensor signal is produced by the interferometry of two out-coupling beams of the same or of differing polarization (with the use of a polarizer). Interferometric measurements are complicated, because the intensities of the two beams have to be matched to one another. In addition, temperature fluctuations due to the interferometric signal generated by differing polarizations (using a polarizer) are only partially compensated.
In SPIE Vol. 2836, 221-234, a layout for a waveguide grating structure in connection with fluorescence or luminescence measurements is described. This layout, however, is not suitable for an (if necessary simultaneous) (absolute) temperature-compensated measurement on the basis of a direct detection. In addition, the waveguide grating structure is mounted on a revolving table.
In Applied Optics 20 (1981), 2280-2283, a temperature-independent optical waveguide is described, whereby the substrate is made of silicon. Silicon is absorbent in the visual spectral range. In the case of chemo-sensitive waveguide grating structures, however, the in-coupling takes place in preference from the substrate side. In addition, Applied Optics 20 (1981), 2280-2283, grating couplers which are not temperature-independent are described.